Window quotes are full of jargon, and installers do not always stop to explain it. This glossary defines the terms you are most likely to meet when buying double glazing in 2026, from the frame materials and window styles to the energy ratings that decide whether a window meets Building Regulations. Each entry is short and factual, so where a definition raises a follow-up it is worth reading the double glazing questions answered alongside it. For the figures behind the words, the cost per window guide sets out 2026 prices, you can build your own estimate in the uPVC window calculator, and the wider queries homeowners bring to a quote are covered in Double Glazing Questions Answered.
Terms A to Z
A-rated glazing
A window with a Window Energy Rating (WER) of band A. A-rated double glazing is the common standard for new replacement windows in 2026. See A-rated vs A+ glass for more.
Aluminium
A powder-coated metal window frame with slimmer sightlines than uPVC and a 30 to 40 year life. It costs roughly 30 to 40% more than uPVC and needs no maintenance beyond cleaning. See our aluminium windows guide for more.
Argon gas
An inert gas sealed between the panes of a double or triple glazed unit. It conducts heat less readily than air, lowering the U-value. See argon vs krypton gas for more.
Bay window
A window that projects outward from the wall in three or more panes, creating a recess inside. More complex and costly to replace than a flat run of windows. See our bay windows guide for more.
Bow window
A window that projects from the wall as a curve of equal-width panes joined at gentle, identical angles, rather than the sharper three-section angles of a bay. A fully fitted uPVC bow window costs £1,250 for three sections rising to £3,400 for six. See our bow windows guide for more.
Building control
The local authority department that checks building work against the Building Regulations. If your installer is not in a competent-person scheme such as FENSA or Certass, you must notify building control yourself and pay a separate fee. See our FENSA guide for more.
Casement window
A window hinged at the side or top that opens outward. The most common and affordable style in UK homes. See our casement windows guide for more.
Conservation area
An area designated by the council for its special architectural or historic interest. Like-for-like window replacement is normally permitted development, but where an Article 4 direction is in force you need planning permission and uPVC is commonly refused on front elevations. See our conservation area rules guide for more.
Double glazing
A sealed unit of two glass panes with a gas-filled gap between them, which cuts heat loss and noise compared with single glazing.
ECO4
The fourth phase of the Energy Company Obligation, under which large energy suppliers fund insulation and heating measures for low-income households. It runs to 31 December 2026 and no successor scheme has been announced. See our ECO4 grant page for more.
EPC rating
The Energy Performance Certificate band, from A (most efficient) to G, required whenever a home is sold or let. Private rentals in England and Wales must reach band E today and band C from 1 October 2030, which is why landlords upgrade glazing. See our EPC C guide for landlords for more.
FENSA
The Fenestration Self-Assessment Scheme. A FENSA certificate proves replacement windows meet Building Regulations without separate council sign-off. See our FENSA guide for more.
Flush casement
A casement window whose opening sash closes level with the outer frame, giving an unbroken flat face instead of the stepped overlap of a stormproof casement. It costs roughly 30% more than an equivalent stormproof casement in the same material and size. See our flush casement guide for more.
Fully fitted
A quote covering the windows plus removal of the old ones, installation, sealing, waste disposal and the FENSA certificate. Fitting adds roughly £300 to £600 a window over the supply-only unit price. See supply-only vs fully fitted for more.
g-value
The proportion of solar heat that passes through the glass, on a scale of 0 to 1, also called solar heat gain or the solar factor. A higher g-value adds free winter heat and lifts the Window Energy Rating band, which is why two windows with the same U-value can rate differently. See our WER ratings guide for more.
Insurance-backed guarantee (IBG)
A separate insurance policy that honours your installer's workmanship guarantee if the installer stops trading. It normally runs for 10 years and should be issued in writing at handover, because the installer's own guarantee is worth nothing once the company folds.
Krypton gas
An inert gas fill that insulates better than argon in a narrow cavity of about 9 to 12 mm. It suits triple glazing and slim heritage units, but costs around 40% more than argon and adds nothing in a standard 12 to 16 mm double glazed cavity. See argon vs krypton gas for more.
Listed building
A building on the national heritage list, graded I, II* or II, whose special character is protected by law. Altering or replacing historic windows normally needs Listed Building Consent and uPVC is generally refused, so secondary glazing at £300 to £900 a window is the usual route. See our listed building glazing costs for more.
Low-E glass
Glass with a microscopically thin metal-oxide coating that reflects heat back into the room while letting light through. It cuts the U-value by around 40%. See our Low-E glass guide for more.
Misting
Condensation trapped between the panes of a sealed unit, which means the seal has failed and the gas fill has escaped. The frame is normally fine, so replacing the glass unit alone costs £55 to £145 for a standard pane. See our condensation guide for more.
Mullion
The vertical bar that divides a window frame into two or more separate lights; the horizontal equivalent is called a transom. A mullion is structural, so it stays fixed when the sashes open and always breaks up the view.
Part L
The part of the Building Regulations covering conservation of fuel and power. Since 15 June 2023 a replacement window in England must achieve a whole-window U-value of 1.4 W/m²K or lower, or a Window Energy Rating of band B or better. See our Building Regs Part L guide for more.
Sash window
A window with one or two panels (sashes) that slide vertically. Traditional in Victorian and Georgian homes. See our sash windows guide for more.
Sealed unit (IGU)
The glass component of a double glazed window: two or three panes bonded around a spacer with an argon or krypton filled cavity between them, also called an insulated glazing unit (IGU). When it mists up the unit alone is replaced for £55 to £145, leaving the frame in place. See our window lifespan guide for more.
Secondary glazing
A second pane fitted to the room side of an existing window, leaving the original frame untouched. It costs £300 to £900 a window against £400 to £1,000 for replacement double glazing, and Historic England recommends it for listed buildings because it is reversible. See secondary vs double glazing for more.
Supply only
Buying the window units without installation, from around £160 for a basic uPVC casement. You get no FENSA certificate, so you must arrange and pay for building control to sign the work off yourself. See supply-only vs fully fitted for more.
Tilt and turn
A window that can tilt inward from the top for ventilation or swing fully open like a door, useful on flats and upper floors. See our tilt and turn guide for more.
Timber
A wooden window frame, the traditional choice for period and conservation-area homes, with a 30 to 60 year life if maintained. It costs roughly 60 to 110% more than uPVC and needs repainting every 8 to 10 years. See our timber windows guide for more.
Trickle vent
A small controllable slot in the window frame or head that provides background ventilation without opening the window. Since the Part F update of 15 June 2022 most replacement windows must include one, typically 8,000 mm² equivalent area for a habitable room and 4,000 mm² for a wet room. See our trickle vents guide for more.
Triple glazing
A sealed unit of three glass panes with two gas-filled gaps. It improves insulation and noise reduction but adds roughly 10 to 20% to the cost. See double vs triple glazing for more.
U-value
A measure of how much heat passes through a window, in W/m²K. Lower is better. Replacement windows must reach 1.4 W/m²K or lower under Part L. See our full U-values guide for more.
uPVC
Unplasticised polyvinyl chloride, the most common and affordable window frame material. Low maintenance with a 20 to 25 year life. See our uPVC windows guide for more.
Warm-edge spacer
The strip that separates the glass panes around the edge of a sealed unit. A warm-edge (low-conductivity) spacer cuts edge heat loss and condensation. See our warm-edge spacer guide for more.
Window Energy Rating (WER)
A British Fenestration Rating Council (BFRC) label rating a window from A++ (best) to G, combining U-value, solar gain and air leakage. Part L (since 15 June 2023) accepts band B or better as one route to compliance for replacement windows, the alternative being a U-value of 1.4 W/m²K or lower. See our WER ratings guide for more.